1. Technical Field
This invention relates to the art of rapid fusion bonding of low density thermoplastic bodies and, more particularly, to the art of making substantially invisible, leakproof joints between such low density thermoplastic bodies.
2. Description of the Prior Art
A commercial need has arisen for bonding together low density thermoplastic bodies as a result of the introduction of the evaporative casting process (ECP). ECP is a process for casting molten metals that uses a plastic foam pattern which is destroyed (evaporated) upon contact with the molten metal. In adapting ECP to the making of large, complex engine components, such as engine blocks or engine heads, the internal surfaces of such pattern cannot be adequately defined unless made in smaller pattern portions which are subsequently joined together. In addition ECP requires that the joint between such pattern portions be leakproof. The bonded plastic assembly is dipped in an aqueous refractory solution to form a thin coating thereon; if such solution penetrates into any voids of the joint, the dried refractory solution will not be replaced by molten metal during the casting process and result in casting defects.
Considerable research has been undertaken to develop improved cost-effective and technically sound means for rigidly bonding such pattern portions together to create a unitary assembly that is not physically inhibited by joints in any way. Early attempts to achieve this goal have included chemical techniques (which are outside the field of this invention) such as solvent bonding (U.S. Pat. No. 4,595,446) and hot melt gluing (U.S. Pat. No. 4,640,728). Solvent bonding is disadvantageous because it creates fumes and is difficult to control. Hot melt gluing is disadvantageous because it (a) does not create an optimally strong bond, (b) adds unwanted organic material that must be vaporized during metal casting, which not only aggravates dimensional control of the casting but contaminates the mould medium, and (c) often creates appearance problems on the final casting due to its messy presence along the joint interior resulting from its low viscosity and runniness.
A recent attempt has been made to effect contact bonding as demonstrated by vibration welding of thermoplastic bodies (U.S. Pat. No. 4,190,093). Although such vibration welding technique has achieved some degree of success in thick cross-sections, it lacks dimensional control in thin wall sections.
Thermal welding of hard, high density plastics has been carried out in the making of automotive lenses and battery cases. Such thermal welding technique is outside the field of this invention because (a) high density thermoplastics retain their melted condition for considerably longer periods of time obviating a need for rapid contact in fusion, and (b) most applications of such high density plastics require a joint which is primarily structural (see U.S. Pat. No. 4,443,288).